Laser cleaning and Raman analysis of the contamination on the optical window of a rubidium vapor cell

In this work, we present the laser cleaning of a Rubidium vapor cell and the Raman analysis of the contaminant material to be removed. The optical window of the vapor cell had gradually lost transparency due to the development of an opaque layer of unknown composition at the inner side during the normal operation of the cell. Laser cleaning was successfully performed by a frequency-doubled Nd:YAG laser focusing the beam inside the cell, avoiding any possible damage to the window. A single laser pulse was enough to clear away the black discoloration at the focal spot and locally restore the transparency of the window. The Raman spectra of the deposit showed peaks not yet described in the literature. Comparison with known Rubidium germanate spectra and simulation results strongly suggested that the unknown material was Rubidium silicate.


Scientific Reports
| (2022) 12:15530 | https://doi.org/10.1038/s41598-022-19645-z www.nature.com/scientificreports/ Its efficiency, low cycle time and material effort coupled with a high level of automation made laser cleaning a lucrative alternative for industrial actors as well. Most commonly, it is used for material preparation before manufacturing in the semiconductor and microelectronic industry, machine industry and waste recycling [23][24][25][26][27][28] . In addition, there are also studies about utilization in the nuclear industry for decontamination purposes 29 .
Process control of the laser cleaning is an essential factor in finding the appropriate irradiation parameters and achieving an efficient layer removal. For historical buildings, visual inspection of the cleaned surface is usually sufficient, but for more demanding samples, a complex approach is necessarily composed of optical microscopy and an appropriate selection of material analysis techniques like Raman spectroscopy, optical reflection spectroscopy, scanning electron microscopy, energy-dispersive X-ray analysis 11,16,22,[37][38][39][40][41]44,45,49,59,60 . Laser-induced breakdown spectroscopy is a promising method to control the ablation during the cleaning process or analyze the surface before it 17,[31][32][33]52,58,61,62 .
Contamination of optical windows of vapor cells (and other optical parts) is a well-known problem originating from these components' operational environment and conditions. The contamination has a negative effect on their optical performance since it can decrease the transmitted laser intensity, modify the wavefront of the laser pulses, and even facilitate laser-induced damage during the operation of the system by creating localized absorption. Most of such contamination consists of particulate material. Earlier studies dealing with the particle contamination of surfaces of optical elements and focusing on identifying the origin of contamination 63 showed that contamination could originate from various sources, including particles from the surrounding environment and products of laser-induced damage of the optics itself or adjacent parts, and therefore can involve various types of materials 64 . Depending on the type of the contamination particle, its interaction with a laser pulse can be characterized by three main effects: particle removal, secondary contamination of the surface by fragments of the original contamination particle, and laser-induced damage of the host surface. The first type of interaction is, in fact, laser cleaning. This effect was used to remove particles from aluminium mirror surfaces with high efficiency by using a pulsed UV laser source 65 . However, there is no example in the literature for removing the rubidium-based layer from a glass substrate, especially not from the inner walls of a closed rubidium cell.
In this work, we present the laser cleaning procedure of a worn rubidium vapor cell together with the Raman analysis of the black discoloration. The cell was used to generate and characterize plasma in rubidium vapors by intense femtosecond laser pulses.

Experimental details
Sample and measurement points. The sample was a worn Rubidium cell from a laser-induced plasma generation experiment, which was no longer in use due to the poor transparency of the exit window (Fig. 1). For the plasma generation, the glass cell filled with rubidium vapor was placed in a vacuum chamber incorporating a temperature-controlled oven. The cell is a cylindrical glass tube of 2.5 cm outer diameter with optical quality quartz end windows. The temperature of the cell and, consequently, the density of the rubidium vapor were varied during the experiments. The plasma was generated by an ionizing Ti-sapphire laser (Legend, Coherent Inc.) operating at 800 nm wavelength and 1 kHz repetition rate, with a pulse duration of about 40 fs and maximum pulse energy of 4 mJ. The laser beam was aligned longitudinally along the glass cell inside the vacuum system, and the transmitted fraction of the laser light was blocked by an appropriate beam dump 66 .
The optical windows of the rubidium vapor cell are made of quartz. While these windows of a new cell are obviously clear, two types of opaque areas can be distinguished on the internal surface of the exit window of the examined worn cell (Fig. 2). The metallic and reddish color areas are of metallic rubidium deposits on the window, which are present in a continuous layer (on the right perimeter of the window, mark no. 3 in Fig. 2) and small droplets (left part, mark no. 5 in Fig. 2). It does not compromise in any form the functionality of the cell since, under operating conditions, the rubidium is in vapor phase and not present on the surface. Furthermore, in the central part of the window, an amorphous discolouration can be observed, which consists of a matte black region (mark no. 1 in Fig. 2) with a grey halo (mark no. 2 in Fig. 2). The exact composition of this layer www.nature.com/scientificreports/ is unknown. However, it is a plausible supposition that during the experiments, the laser pulses heated/ablated the quartz material and the emitted material interacted with rubidium forming some kind of rubidium silicates, especially if we consider the fact that the laser irradiation of the quartz window was the most intense at the area where this surface alteration occurred. Last but not least, a large intact quartz surface is present (mark no. 4 in Fig. 2), which offers an ideal location for reference measurements.
Laser cleaning. The laser cleaning tests of the colored surface were performed by a Quantel Brilliant Q-switched Nd:YAG laser operating at its fundamental wavelength (1064 nm). The pulse width was 3.2 ns (FWHM). Various pulse energies were used during the experiments, cautiously increasing the energy from 50 to 360 mJ (maximum output of the laser in question) by setting the time delay between the flash lamp and Q-switch. The laser beam profile was Gaussian with a beam diameter of 5 mm. The laser radiation was passing through the intact window of the cell, focused by a biconvex converging lens of 295 mm focal length to a point 1 mm in front of the contaminated surface (at the inner side of the window) to minimize heat stress to the glass material and prevent the formation of micro-cracks threatening with the destruction of the sample (Fig. 3). The laser was operated in single pulse mode for the same reason.
The fluence can be calculated to 400 J/cm 2 (1.25 × 10 11 W/cm 2 ), assuming ideal focusing conditions and considering the 50 mJ pulse energy, 5 mm beam diameter, 295 mm focal length with 1 mm defocusing. With 360 mJ (maximum pulse energy), the fluence is around 3 kJ/cm 2 (9 × 10 11 W/cm 2 ). These values are generally enough to ignite weak microplasmas, thus triggering shockwaves to boost cleaning efficiency and possibly enable LIBS analysis.
The irradiated surface was placed perpendicular to the laser beam. Scanning the sample in 2D was performed by a manual stage, keeping the defocusing constant and discharging only one pulse at one point of the sample.
A Stellarnet Bluewave compact Czerny-Turner spectrographs were coupled to the laser to perform LIBS analysis for enhanced control of the cleaning process if the laser irradiation generated microplasma (Fig. 3).

Raman analysis. Raman spectroscopic measurements were performed on a Horiba Jobin Yvon portable
Micro-Raman spectrometer with 532 nm excitation. The excitation laser beam was focused onto different areas of the internal surface of the optical window from the outside by using a Nikon 50× objective lens (NA = 0.75). The Raman spectra were recorded in the 200-1000 cm −1 region with 6 cm −1 resolution.
The self-consistent density functional theory (DFT) field method using the hybrid B3LYP functional consisting of a linear combination of the pure corrected exchange functional by Becke 67 and the three-parameter gradient-corrected correlation functional by Lee et al. 68 was applied for geometry optimizations of these clusters and Raman spectra calculations using the Gaussian-09 program package 69 . The Los Alamos National Laboratory (LANL) double zeta (DZ) valence basis set (LANL2DZ) was used for all atoms in the cluster models 70 . The vibrational contribution of saturating hydrogen atoms in the calculated Raman spectra of Rb 2 Si 2 O 8 H 6 , Si 3 O 9 H 6 and Rb 2 Si 3 O 10 H 6 clusters were eliminated as described elsewhere 71,72 . The Lorentz-shape function with the intensity proportional to the calculated Raman activity and with full width at half-height (FWHH) of 10 cm −1 was used to simulate the Raman spectra of clusters.

Results and discussion
Removal of the residues. The minimum laser pulse energy was applied, targeting the black surface at point 1 in Fig. 2. It was enough to restitute the window's transparency in a spot of 0.6 mm diameter (Fig. 4).
Observing no damage to the window, the fluence was gradually increased from the minimum value to the maximum (roughly 3 kJ/cm 2 , as discussed above). During the laser irradiation, the coloration of the window www.nature.com/scientificreports/ disappeared, which became transparent anew. The cleaned and the intact surfaces showed no difference at optical microscope inspection. During the process, no microplasma generation was observed.
Considering that the window is practically fully transparent to the laser radiation after the ablation of the surface contamination, the absorbed energy density is probably much less. It explains why no plasma was formed by experimental parameters usually suitable for LIBS measurements.
Composition of the contamination. Raman spectroscopic measurements were performed on the black central and grey outer part of the contamination, the rubidium layer and the clean glass (marked with X signs in Fig. 2) to determine the origin of the contamination in the central part of the optical window. Several Raman spectra were recorded in each area that was found to be characteristic of all four spots. These spectra are compared in Fig. 5. The spectrum of the clear glass consists of broad peaks with a high-intensity asymmetric band in the 200-600 cm −1 region. The Raman peaks observable in this spectrum are characteristic of fused silica. They can be attributed to the vibrations of oxygen atoms within the silica tetrahedra (at 440 cm −1 ), breathing vibrations of oxygen atoms in 4-membered rings (at 490 cm −1 ) and 3-membered rings (610 cm −1 ), respectively, various bending and stretching motions (at 793 cm −1 ) and stretching involving SiO 4 tetrahedral units and/or  On the other hand, the Raman spectrum of the contamination is remarkably different (grey and black spots in Fig. 6). A strong and narrow peak dominates the curves of both the grey and black regions at 278 cm −1 . Additional new bands of smaller intensity can also be detected at 350, 460, 556, 910 and 1045 cm −1 . From these new peaks, the 350, 910 and 1045 cm −1 ones are present in the spectrum of the black spot only. The characteristic Raman features of the fused silica can also be observed, serving as a background for the new peaks. However, the shape of some bands differs from that observed in the clean spot spectrum, indicating the glass's structural transformation in the contamination area.
The contaminant was formed under high-energy pulsed laser radiation in a closed cell filled with rubidium vapor. Therefore, it can be assumed that the grey and black discolored residues consist of the material inside the cell, namely quartz and/or rubidium-moreover, the easy contamination removal by a cca. 3 kJ/cm 2 nanosecond pulsed laser indicates that it is a particulate material aggregated on the inner surface of the quartz window (and not the result of the incorporation of the metal into the glass), and the new Raman peaks arise from this structure.
The effect of alkali cations on Raman spectra and the structure of SiO 2 glasses were investigated earlier 75   www.nature.com/scientificreports/ reported. In general, it was found that the bands in the 900-1200 cm −1 spectral region result from highly localized Si-nonbridging oxygen stretching modes, allowing to determine the alkali distributions around SiO 4 tetrahedra. The Raman intensity of the low frequency spectral bands at 500 and 600 cm −1 in the Raman spectra of rubidium-silicate glasses are found to increase with increasing of Rb content. First band was assigned to highly delocalized vibrational mode of both bridging and non-bridging oxygens in the silicate network, while the second band can originate from a vibrational mode localized on a defect structure 75 . The highly localized vibrational mode at 950 cm −l in the Raman spectra of rubidium disilicate glass result from Si-O stretching in SiO 4 tetrahedral units containing two non-bridging oxygens. The spectral features at 1100 and 1150 cm −1 in the Raman spectra of rubidium-silicate glasses indicate the presence of two distinct structural environments in which an SiO 4 tetrahedron contains one non-bridging oxygen 75 .
The features observed in the Raman spectrum of the contaminant can also be compared with the Raman spectra of the xRb 2 O(1 − x)GeO 2 rubidium germanate system 74 , since the structure of silicate (quartz) glasses is similar to that of germanates (both are composed of tetrahedral-silicate and germanate, respectively). There, three peaks were reported at 250, 370 and 512 cm −1 , the intensity of which increases with Rb 2 O content. They were attributed to the Raman active bending modes of Q 2 , and Q 1 tetrahedra of the germanate, which become localized as the number of non-bridging oxygen atoms increases and the germanate tetrahedron starts acquiring molecular-like character. The appearance of these new peaks was also accompanied by a redistribution of Raman intensities in the 720-870 cm −1 region, where a broad band can be attributed to Ge-O − vibrations of different Q 0 -Q 3 germanate tetrahedrons. The shift of the Raman bands towards lower wavenumbers in this region was attributed to the decrease of Q 3 and Q 2 type tetrahedrons and the increase of the Q 1 and Q 0 species with rubidium content 74 . All these structural transformations were attributed to the incorporation of the rubidium oxide into the glass network.
Features similar to those described for xRb 2 O(1 − x)GeO 2 can be observed in the Raman spectra recorded in different areas of the quartz window of the worn vapor cell. A comparison of the Raman spectrum of the contamination with that of the clear glass in our vapor cell shows the appearance of the 278, 350 and 545 cm −1 bands, accompanied by changes in the 900-1100 cm −1 region belonging to different Q 0 -Q 3 species of the silicate tetrahedra. Although the peak positions are different from those observed in rubidium-germanate (mainly because of the presence of heavier Ge in the latter instead of Si, shifting the Raman peaks to lower wavenumbers), the similar changes in the Raman spectra imply the incorporation of rubidium into the silicate structure causing the increase of the amount of non-bridging oxygen and formation of molecular-like Q 2 and Q 1 tetrahedra. So the black contaminant is a silicate glass containing rubidium, presumably in oxide form.
In order to verify this interpretation of the Raman features, the molecular modeling of different gas-phase silicon-oxide, rubidium-silicon, rubidium-oxide and rubidium-silicon-oxide clusters was performed (Fig. 7). Figure 8 show the comparison of experimental Raman spectra of rubidium cell measured in the regions of clean SiO 2 glass (curve 1) and contaminated black spot (curve 2) together with the simulated Raman spectra of different clusters (curves 3-8). The experimental Raman spectra of tridymite polymorph of silica is also shown for comparison (curve 9). As can be seen, the vibrations of Rb 4 Si 4 and Rb 2 O 9 clusters can contribute to the low frequency region of experimental Raman spectra of black spot. Two Raman active modes at 208 and 362 cm −1 were calculated for Rb 4 Si 4 cluster. The main characteristic Raman mode of rubidium suboxide (Rb 2 O 9 cluster) is located at 239 cm −1 (Fig. 8, curves 3 and 4). Therefore, the two bands in the experimental Raman spectra at 278 and 360 cm −1 can be related with the presence of structures based on Rb-Si and Rb-O clusters. However, the Rb 4 Si 4 and Rb 2 O 9 clusters have no high frequency vibrational modes observed in the experimental Raman spectra.
The high frequency Raman modes can be related with the entire transformations of silica network surface induced by Rb ions. Therefore, the additional cluster models were used to verify this idea. First, the isolated SiO 4 anion was used and the Raman active modes of this cluster were calculated at 310, 436 and 582 cm −1 (Fig. 8, curve 5). Then, the two SiO4 tetrahedra each with one non-bridging oxygen were used to model the Rb incorporation in the glass structure (cluster Rb 2 Si 2 O 8 H 6 ). As can be seen, the calculated Raman modes of this cluster at 696 and 1036 cm −1 are in good accordance with the experimental bands at 705 and 1045 cm −1 observed in the Raman spectra of black spot (Fig. 8, curves 2 and 6). In addition, the model built from three corner sharing SiO 4 tetrahedra were used to study the influence of Rb atoms on the structure and vibrational properties of simplest 6-membered ring. The calculated Raman modes of Si 3 O 9 H 6 cluster at 524/556 and 906 cm −1 agree well with the experimental bands at 556 and 910 cm −1 observed in the Raman spectra of black spot (Fig. 8, curves 2 and 7). The presence of non-bridging oxygens (O − charged defects) in the ring model can lead to the incorporation of Rb cation (cluster Rb 2 Si 3 O 10 H 6 ). The main Raman active modes of this cluster calculated at 532 and 1010 cm −1 are in good agreement with the experimental bands at 556 and 1045 cm −1 (Fig. 8, curves 2

Conclusion
It has been demonstrated that laser cleaning is an efficient tool for removing the contamination of the Rubidium vapor cells. The transparency of the optical window contaminated on the internal surface inside the closed cell was successfully restored with a single shot of a frequency-doubled Nd:YAG laser without inflicting any damage to the window itself. The Raman analysis of the contaminant displayed peaks, which had not been described in the literature before. Comparison of the Raman spectra of the sample with reference spectra together with calculated Raman spectra of model structures showed that the material of the discoloration consisted of Rubidium silicates formed upon irradiation of the rubidium vapor with high-intensity femtosecond laser pulses.

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.  (9) is also shown for comparison 76